The present invention relates to a golf ball made of a core and a cover of one or more layer encasing the core. More particularly, the invention relates to an improved golf ball in which the degree of crosslinking due to urethane bonds is varied between an outside portion and an inside is portion of the cover, thereby endowing the ball with an excellent scuff resistance, excellent spin properties, and an excellent feel on approach shots, resistance to discoloration and productivity.
The use of polyurethane materials as golf ball cover-forming materials in recent years is noteworthy. Polyurethane materials, from the standpoint of the molding method used to obtain moldings therefrom, are broadly divided into thermoset polyurethane materials and thermoplastic polyurethane materials.
However, moldings of thermoset polyurethane materials have no plasticity when heated, and so the feedstock and molded articles made therewith cannot be recycled. Moreover, in the production of such moldings, the thermosetting step and the cooling step take a long time, in addition to which the feedstock has a high reactivity when heated and is thus unstable, making the molding time very difficult to control. Hence, the productivity of thermoset polyurethane materials when used to make special moldings such as golf ball covers (moldings which encase a core material) is regarded as poor.
By contrast, moldings of thermoplastic polyurethane materials are not obtained by the direct reaction of a feedstock; instead, a linear polyurethane material synthesized by using starting materials and a production process which differ somewhat from the foregoing thermoset polyurethane materials are used in molding. Such polyurethane materials are thermoplastic; thermoplasticized polyurethane materials have the quality of hardening on cooling. Therefore, it is possible to mold such polyurethane materials using an injection molding machine. The injection molding of thermoplastic polyurethane materials is ideal as a method of molding golf ball covers, both because the molding time is very short compared with the molding time for thermoset polyurethane materials and because this method is suitable for precision molding. Also, thermoplastic polyurethane materials are recyclable and thus friendly to the global environment.
JP-A 2002-336378 describes a golf ball obtained using a cover material composed of a thermoplastic polyurethane material and an isocyanate mixture. The cover material is a thermoplastic polyurethane material that is recyclable and moreover has a high resilience and an excellent scuff resistance. This cover material makes it possible both to achieve the good productivity of a thermoplastic polyurethane and to exhibit physical properties comparable with those of a thermoset polyurethane; at the same time, due to the plasticizing effect by the isocyanate compound, such an approach enhances the flow properties of the thermoplastic polyurethane material and is thus able to improve productivity. Although this art is outstanding in the above respects, because burn contaminants arise due to direct charging of the isocyanate mixture into the molding machine and there is some variability in the compounding ratio owing to the use of dry blending, the uniformity is poor, giving rise to molding instability. At the same time, the compositional ratio within the isocyanate mixture between the isocyanate compound and the thermoplastic resin that is substantially non-reactive with isocyanate has already been set, and so one has less freedom of choice in the amounts and types of isocyanate compound and thermoplastic resin to be added.
JP 5212599 describes a golf ball which has a high rebound and an excellent spin performance and scuff resistance, and also has a high cover material flowability and a high productivity. However, the thermoplastic polyurethane material used in this art is a special resin mixture, and there are challenges regarding the production, supply and cost of such a resin mixture. Moreover, because this art entails charging an injection molding machine and ancillary equipment with a material in which isocyanate groups remain in an unreacted state, undesirable effects such as seizing and solidification due to deposition of the isocyanate ingredient arise and can cause an increase in the percent defective.
JP 3494441 discloses art where, in a golf ball having a cover made of thermoplastic resin, the surface layer of the cover is subjected to modification treatment, thereby giving a cover having excellent properties. However, although this is an excellent approach that enables the properties to be modified after the cover has been molded of a thermoplastic material of excellent moldability, there remains room for improvement in the degree to which the scuff resistance is enhanced and in the golf ball properties following modification. Particularly in cases where 4,4′-diphenylmethane diisocyanate (MDI) has been selected as the polyisocyanate compound, the MDI must be held at the solidifying point (about 39° C.) or higher in order to place it in a molten liquid state suitable for treatment. In the case of MDI, production of the dimer also requires care; that is, dimer production generally speeds up at a higher temperature, and so close temperature control is required to ensure stable productivity, which can be troublesome. Moreover, in cases where an ionomer material is used in part of the golf ball, there is also a possibility that treatment at a high temperature will give rise to deformation or a change in properties.
JP 4051374 describes a method of manufacturing golf balls produced with a thermoplastic polyurethane, polyurea or mixed material thereof having a high melt index. More specifically, this method includes the step of obtaining a base thermoplastic polyurethane, polyurea or mixed material thereof; the step of raising the melt index of this base resin material; the step of molding the thermoplastic polyurethane material having an increased melt index in a mold; and the subsequent step of treating the molded material with a secondary curing agent such as an isocyanate solution. This art is outstanding in that, by crosslinking the cover material using an isocyanate ingredient, resistance to damage can be imparted while maintaining the other desirable characteristics of the cover, such as a soft feel. However, in cases where the secondary curing agent includes an organic solvent capable of causing the cover material to swell, depending on the treatment conditions, this may cause deformations on the order of several microns in the dimple shape, which may adversely impact the flight performance, and the appearance of the golf ball may worsen due to solvent infiltration to the cover material interface that arises during molding.